Buggy code:
final class InvocationFuture < E > implements InternalCompletableFuture < E > { private static final int MAX_CALL_TIMEOUT_EXTENSION = 60 * 1000 ; private static final AtomicReferenceFieldUpdater < InvocationFuture , Object > RESPONSE_FIELD_UPDATER = AtomicReferenceFieldUpdater . newUpdater ( InvocationFuture . class , Object . class , "response" ) ; private static final AtomicIntegerFieldUpdater < InvocationFuture > WAITER_COUNT_FIELD_UPDATER = AtomicIntegerFieldUpdater . newUpdater ( InvocationFuture . class , "waiterCount" ) ; volatile boolean interrupted ; private volatile int waiterCount ; private final OperationServiceImpl operationService ; private final Invocation invocation ; private volatile InvocationFuture . ExecutionCallbackNode < E > callbackHead ; private volatile Object response ; InvocationFuture ( OperationServiceImpl operationService , Invocation invocation , final Callback < E > callback ) { } static long decrementTimeout ( long timeout , long diff ) { } @ Override public void andThen ( ExecutionCallback < E > callback , Executor executor ) { } @ Override public void andThen ( ExecutionCallback < E > callback ) { } private void runAsynchronous ( final ExecutionCallback < E > callback , Executor executor ) { } public void set ( Object offeredResponse ) { assert ! ( offeredResponse instanceof Response ) : "unexpected<seq2seq4repair_space>response<seq2seq4repair_space>found:<seq2seq4repair_space>" + offeredResponse ; if ( offeredResponse == null ) { offeredResponse = InternalResponse . NULL_RESPONSE ; } InvocationFuture . ExecutionCallbackNode < E > callbackChain ; synchronized ( this ) { if ( ( ( response ) != null ) && ( ! ( ( response ) instanceof InternalResponse ) ) ) { ILogger logger = invocation . logger ; if ( logger . isFinestEnabled ( ) ) { <START_BUG> logger . info ( ( ( ( ( ( "Future<seq2seq4repair_space>response<seq2seq4repair_space>is<seq2seq4repair_space>already<seq2seq4repair_space>set!<seq2seq4repair_space>Current<seq2seq4repair_space>response:<seq2seq4repair_space>" + ( response ) ) + ",<seq2seq4repair_space>Offered<seq2seq4repair_space>response:<seq2seq4repair_space>" ) + offeredResponse ) + ",<seq2seq4repair_space>Invocation:<seq2seq4repair_space>" ) + ( invocation ) ) ) ; <END_BUG> } return ; } response = offeredResponse ; if ( offeredResponse == ( InternalResponse . WAIT_RESPONSE ) ) { return ; } callbackChain = callbackHead ; callbackHead = null ; notifyAll ( ) ; } operationService . invocationsRegistry . deregister ( invocation ) ; notifyCallbacks ( callbackChain ) ; } private void notifyCallbacks ( InvocationFuture . ExecutionCallbackNode < E > callbackChain ) { } @ Override public E get ( ) throws InterruptedException , ExecutionException { } @ Override public E getSafely ( ) { } @ Override public E get ( long timeout , TimeUnit unit ) throws InterruptedException , ExecutionException , TimeoutException { } private Object waitForResponse ( long time , TimeUnit unit ) { } private void pollResponse ( final long pollTimeoutMs ) throws InterruptedException { } long getMaxCallTimeout ( ) { } private static long getCallTimeoutExtension ( long callTimeout ) { } int getWaitingThreadsCount ( ) { } private static long toTimeoutMs ( long time , TimeUnit unit ) { } private Object resolveApplicationResponseOrThrowException ( Object unresolvedResponse ) throws InterruptedException , ExecutionException , TimeoutException { } private Object resolveApplicationResponse ( Object unresolvedResponse ) { } @ Override public boolean cancel ( boolean mayInterruptIfRunning ) { } @ Override public boolean isCancelled ( ) { } @ Override public boolean isDone ( ) { } @ Override public String toString ( ) { } private static final class ExecutionCallbackNode < E > { private final ExecutionCallback < E > callback ; private final Executor executor ; private final InvocationFuture . ExecutionCallbackNode < E > next ; private ExecutionCallbackNode ( ExecutionCallback < E > callback , Executor executor , InvocationFuture . ExecutionCallbackNode < E > next ) { } } private static final class ExecutorCallbackAdapter < E > implements ExecutionCallback < E > { private final Callback callback ; private ExecutorCallbackAdapter ( Callback callback ) { } @ Override public void onResponse ( E response ) { } @ Override public void onFailure ( Throwable t ) { } } }

Fix:
logger . finest ( ( ( ( ( ( "Future<seq2seq4repair_space>response<seq2seq4repair_space>is<seq2seq4repair_space>already<seq2seq4repair_space>set!<seq2seq4repair_space>Current<seq2seq4repair_space>response:<seq2seq4repair_space>" + ( response ) ) + ",<seq2seq4repair_space>Offered<seq2seq4repair_space>response:<seq2seq4repair_space>" ) + offeredResponse ) + ",<seq2seq4repair_space>Invocation:<seq2seq4repair_space>" ) + ( invocation ) ) ) ;